Sensors & Measurement (ENT 164)
Laboratory Module
EXPERIMENT 3 (b)
SWITCHES
1. OBJECTIVES:
1.1
1.2
1.3
To understand the characteristics of a reed switch and its application
To experiment the applications of a mercury switch
To experiment the characteristics of a limit switch
2. COMPONENTS & EQUIPMENTS:
1 DC power supply
1 Sensor Unit SU-6810
1 Multimeter
3. INTRODUCTION:
Switching is part of a sensor action for many devices. Furthermore, switches, as
an electromechanical component, are a vital part of any sensing or transducing
equipment.
Switches are almost as old as present-day electricity applications. The function of
a switch is to make or break current in a circuit. Early switches were simple mechanical
devices, the knife switches that can still be seen in old factories. This century has seen
the development of much more elaborate mechanical principles and of switches with no
mechanical moving parts at all.
The switch, then, is a device with a circuit path that can be made or broken.
When the circuit path is made, the circuit resistance through the switch is low, of the
order of mΩ for most switches. With the circuit path broken, the circuit path through the
switch has a high resistance, several MΩ or higher. This resistance, and the maximum
voltage that can be applied to this insulation, is often of major importance in mains
voltage switches, and is vital feature of legislation regarding switch suitability.
A reed switch has two magnetic contacts placed inside a glass which is sealed
with some type of inert gas or vacuum, as in Figure 3.1 (b). When a magnet is brought
close to the switch, the contacts which are made out of magnetic material attract to each
other. Sometimes, the reed contacts are surrounded by a wound coil as shown in Figure
3.1 (c). In this case, a DC current is needed to close the contacts. This type of device is
called a reed relay. A reed switch and a magnet are commonly used as a proximity
sensor. Some of the characteristics of reed switch that has brought to its advantages are
as follows:
•
•
•
•
Performance not critical to the ambient temperature or humidity
Good contact reliability
Low cost
Quiet operation without contact chattering
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•
Laboratory Module
Life is extremely long due to the existence of vacuum or gas
Figure 3.1: Reed switch and reed relay
A mercury switch makes use of the property that the element mercury has – its
good electrical conduction and high surface tension. A drop of mercury is placed inside a
glass tube with two metal contacts separated with each other. As the tube is at level, the
mercury forms a bridge between two metal points. However, when the tube is at an
angle, the mercury drifts to one corner and the two points are no longer electrically
connected. Mercury switches are used in level control applications, and as a thermostat
on the wall. The mercury contact is small in size, but can handle a large amount of
current.
(a) A mercury switch
(b) On / off operation of a mercury switch
Figure 3.2: Construction of a mercury switch
Meanwhile, limit switches are similar to a microswitch. When a sensing lever is
pressed by a motion, the contacts inside closed. No power is needed to operate limit
switches. Contacts can handle large amount of currents. Moreover, its on/off operation is
more precise than a regular push button switches. Limit switches are normally being
used in interlock circuits and safety switches in an electrical or a mechanical machine.
Due to its availability in different types, it is important to choose the right kind of limit
switch for the given application.
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(a) Roller Lever Type
Laboratory Module
(b) Lever Type
(c) Structure
Figure 3.3: Limit switches
4. PROCEDURE:
4.1 Reed Switch Applications
1. Prepare a power supply (± 15 V) and keep the power off. Make connections as
shown in Figure 3.4. Set the multimeter to R X 10 Ω range.
2. Turn the power on, and set the Speed adjust at lower right of the sensor unit to
mid position.
3. Using the CW and CCW switches, confirm the ON/OFF operation of the reed
switch. Adjust the spacing between the reed switch and magnet. Record the
values for various operational distances in Table 3.1. What is the maximum
operational distance between the contact and magnet?
Figure 3.4
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4.2 Mercury Switch
1. Take out the mercury switch carefully out of the Sensor unit. Using a multimeter,
experiment the on/off action of the mercury switch by recording its value of
resistance and record them in Table 3.2.
2. When experiment is done, make sure the switch is properly re-installed in the
Sensor unit.
4.3 Limit Switch
1. With the power off, make connections as shown in Figure 3.5. Set the multimeter
to R X 10 Ω range.
2. Turn the power on, and set the Speed adjust at lower left to mid position.
3. Press CW or CCW switches, and observe the limit switch turning on or off.
Record the value of resistance in Table 3.3.
4. Then, change the multimeter connection from NO pin to NC pin, with COM pin
unchanged. Observe the differences. Record the resistance values in Table 3.3.
Figure 3.5
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Name:____________________________ Matrix No.:_____________ Date:_________
5. RESULTS
Table 3.1
Distance (mm)
Resistance (Ω)
3
6
9
12
15
Table 3.2
Situation
Resistance (Ω)
Mercury touches switch
Mercury away from switch
Table 3.3
Normally Open (NO)
Resistance when
Resistance when
touch (Ω)
untouched (Ω)
Normally Closed (NC)
Resistance when
Resistance when
touch (Ω)
untouched (Ω)
Instructor Approval: ………………………………………….
Date: ………………
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Name:____________________________ Matrix No.:_____________ Date:_________
6. DISCUSSION
7. CONCLUSION
Instructor Approval: ………………………………………….
Date: ………………
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Laboratory Module
Name:____________________________ Matrix No.:_____________ Date:_________
PROBLEMS
1. Why platinum is preferred for the industrial resistance thermometers application?
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2. A thermistor has constants K = 0.25 Ω, β = 4125 K. Find the resistance at the ice
point.
Instructor Approval: ………………………………………….
Date: ………………